The manufacturing industry, and particularly the automotive industry, has increasingly been replacing ferrous materials with light weight materials such as aluminum. The demand for substitute light weight materials has led to the development of aluminum alloys capable of forming structures that will withstand stresses typically reserved for structures formed from ferrous metals. In addition to enhanced strength (including both high yield strength and high elongation values) an aluminum alloy should be die-castable, corrosion resistant, and readily machinable.
Historically, aluminum castings have been characterized by relatively low strength and ductility compared to wrought products of similar compositions. This low strength and ductility is due to the presence of defects in cast alloys which are largely eliminated by mechanical working in wrought alloys. These defects are chiefly of two types: voids due to shrinkage or gas inclusions, and rather large brittle particles due to intermetallic phases formed from impurity elements or oxide inclusions trapped in the casting during solidification. The development of higher quality castings results from changes in alloy composition and casting techniques designed to minimize the number and size of these defects.
The highest quality aluminum casting alloys, in most part, fall into the Aluminum/Silicon/Magnesium (Al-Si-Mg) type of alloy. Enhanced strength and ductility is achieved chiefly by using high purity input (low iron content and/or modification of AlSiFe.sub.5 by Beryllium (Be) additions) as well as keeping the alloy clean. As a consequence of these changes, properties of presently available aluminum castings can approach those of wrought products of equivalent composition. However, there remains a need for an aluminum alloy having further enhanced mechanical properties. The aluminum based alloy of the present invention has substantially improved yield strength and elongation values over currently available aluminum alloys.
The effects of various elements on the mechanical properties of aluminum alloys have been studied, however, the investigations have been conducted mostly on relatively simple systems, binary or ternary alloys. Most commercial aluminum die casting alloys are complex alloy systems containing several alloy and impurity elements. The large number of elements encountered in these alloys, their low, varying concentrations and the possibility of interactions between the alloy elements, makes the systematic study of the effect of the individual elements on the properties of commercial alloys very complicated and difficult. Regardless of the difficulty in deciphering the effects individual elements have on an alloy's mechanical properties, magnesium, manganese, iron, silicon, and beryllium are accepted by skilled practitioners as having the following general effects on aluminum alloy properties:
Magnesium is typically incorporated to enhance the tensile strength of the alloy. Al-Mg binary alloys have high strength, excellent corrosion resistance, weldability and surface finish. However, while increased magnesium content enhances the hardness and fatigue resistance of the alloy, it also decreases the alloy's ductility. An additional reason for limiting magnesium content in the alloy is that magnesium can easily oxidize to form magnesium oxide (MgO) microsized particles within the melt. At high holding temperatures (greater than 750.degree. C.) spinel, which is a complex aluminum magnesium oxide, usually forms and grows rapidly forming inclusions in the melt. These inclusions reduce the fluidity and elongation properties of the alloy.
Copper can also be added to an aluminum alloy to increase the strength of the alloy. As copper content increases, hardness of the alloy increases, but strength and ductility depend on whether the Cu is in solid solution, or as spheroidized and evenly distributed particles. Copper decreases the electrolytic potential, and also the corrosion resistance. Copper bearing alloys tend to pit severely in the annealed condition and when age hardened may be susceptible to intergranular or stress corrosion.
Silicon is an important component of the alloy for the purpose of improving the flowability of the alloy in a molten state during the course of the die casting operation. Al-Si alloys have low shrinkage and narrow freeze range resulting in their good hot tear resistance, soundness and good weldability. Silicon in Al-Mg alloys reduces ductility and elongation without a compensating increase in strength. The combined introduction of copper and silicon significantly increases the hardness of alloy but sharply reduces the elongation.
Iron is typically added to die casting aluminum alloys for the purpose of preventing the aluminum alloy from sticking to a metal die during the course of the die casting operation and enhancing the release of the aluminum alloy from the die. However, the addition of iron will lower the elongation of the aluminum alloy. Manganese is added to aluminum alloys for the purpose of eliminating the adverse effect of the addition of iron. However, an excess of manganese can result in a lowering of the mechanical strength of the aluminum alloy.
Beryllium is added to Al-Mg based alloys to prevent oxidation of the magnesium content of the aluminum alloy. As little as 0.005% to 0.05% by weight beryllium added to an aluminum based alloy melt causes a protective beryllium oxide film to form on the surface. Without the protection that beryllium provides, significant magnesium losses can occur during casting because magnesium is highly reactive to oxygen. Magnesium oxide by itself does not form a protective barrier to prevent magnesium loss. Beryllium has also been included in aluminum alloys to enhance the corrosion resistance, elongation and strength of aluminum alloys. Therefore in accordance with the current state of the art, beryllium is routinely included in Al-Mg alloys; the percentage of beryllium varying with the magnesium content of the aluminum alloy.
Contrary to the presently accepted teaching regarding the benefits of including beryllium in Mg-Al alloys, applicants' have discovered that the mechanical properties of a Mg-Si-Al alloy can be enhanced by lowering beryllium content below 0.003% by weight, including entirely eliminating Be from the alloy.
Berylliosis, a chronic irreversible lung disease, has occurred among workers engaged in the production of beryllium containing alloys. By breathing air contaminated with small particles of beryllium, a person is subjected to the risk of contracting berylliosis. Thus an additional benefit derived from eliminating beryllium from applicants' aluminum alloy is the decreased exposure to beryllium and the prevention of berylliosis.
Applicants' present invention is directed to a die casting aluminum alloy having improved elongation and comprising 2.5-4.0% by weight magnesium, a maximum of 0.4% by weight manganese, a maximum of 0.6% by weight iron, a maximum of 0.45% by weight silicon, a maximum of 0.10% by weight copper, less than 0.003% by weight beryllium with the remainder being aluminum. This aluminum alloy is useful for forming light weight die cast articles having superior elongation over die cast articles formed from currently available aluminum die cast alloys.